1. Technical Field
The invention relates to the measurement of characteristics of optical paths and is especially, but not exclusively, applicable to the measurement of polarization-dependent characteristics of optical fibers.
2. Background Art
In optical communication systems, newly-installed optical fibers generally have low levels of, for example, polarization mode dispersion (PMD) and can handle current high bit rates. Optical fibers which have been installed for several years, however, may exhibit levels of PMD that are unacceptable for modern optical communication systems. It has been found that, in many cases, the unacceptable overall PMD is caused by a short section of the optical fiber cable. It would be desirable, therefore, to be able to determine which short sections have the worst PMD, and replace those sections only. It has been proposed to use so-called polarization-optical time domain reflectometry (P-OTDR) to locate such sections.
P-OTDR is predicated upon the fact that, although conventional optical time domain reflectometers (OTDRs) measure only the intensity of backscattered light to determine variation of attenuation along the length of a transmission path, the backscattered light also exhibits polarization dependency. P-OTDR utilizes this polarization dependency to monitor polarization dependent characteristics of the transmission path, e.g., an installed optical fiber.
The concept of P-OTDR was introduced in the early 1980s by Rogers [1], who described an OTDR sensitive to the state of polarization (SOP) of the backscattered signal. The simplest P-OTDR comprises an OTDR having a polarizer analyzer in the return path, just prior to its detector. Although initially developed as part of a fiber sensor system for monitoring spatially varying external physical parameters (temperature, strain, etc.), there has recently been heightened interest in variants of this approach to measure the distributed PMD [2-4].
U.S. Pat. No. 5,384,635 (Cohen) discloses a variation based upon synchronous detection to detect cyclic physical perturbations or vibrations of the fiber. More recently, U.S. Pat. No. 6,229,599 (Galtarossa), which is incorporated herein by reference, discloses apparatus for measuring beat length, correlation length and polarization mode dispersion at different positions along the length of the optical fiber. One limitation of Galtarossa""s technique is that it derives statistics based upon wavelength and so needs a wavelength tunable source.
Each of these known techniques requires that the P-OTDR have sufficient spatial resolution to xe2x80x9cseexe2x80x9d the evolution of the SOP as the light propagates down the fiber. This entails the use of short pulses since, when the birefringence of the fiber is large ( greater than 1 ps/km), the backscattered beat length is short ( less than 2.5 m) and a short P-OTDR pulse must be used (10 nsec or less). The higher the birefringence of the fiber, the shorter must be the pulse of the P-OTDR. Shorter P-OTDR pulses imply a smaller dynamic range for the instrument. Therefore, high-PMD fibers, that necessarily exhibit high birefringence, are more difficult to characterize than low-PMD fibers.
The PMD of an optical fiber depends upon both the birefringence xcex2 and the coupling length h through the following approximation:   PMD  =            β      ⁢              xe2x80x83            ⁢      L                      L        h            
where L is the length of the fiber. Generally, the coupling length, h can be defined as the distance required for a significant portion of energy in one mode (fast or slow) to be transferred to another mode. When coupling length h is short, there is a considerable amount of xe2x80x98scramblingxe2x80x99 between the fast and slow axes and the total PMD for the fiber increases proportionally to the square root of the fiber length ({square root over (L)}). In contrast, if coupling length h is very long, there is very little coupling between the fast and slow axes and the PMD increases linearly with distance (L).
Fibers that have very little coupling between fast and slow axes (long coupling length h) most likely will exhibit high PMD values, since PMD will accumulate more rapidly with distance. Therefore, the detection of a long coupling length h should allow the identification of most of the high PMD sections in a fiber link.
The distribution of coupling length h along the fiber may be determined by making a fully polarimetric measurement of the SOP as a function of distance. Although this can be achieved via several different P-OTDR implementations, a simple approach is to use a rotatable quarter-wave plate followed by a polarizer prior to the P-OTDR detector. The polarimetric SOP information (the four Stokes parameters S0, S1, S2 and S3) is obtained by taking four different P-OTDR traces, with an appropriate orientation for the quarter-wave plate and the polarizer of the analyzer for each trace. Each trace represents intensity of the backscatter signal against distance for the corresponding one of the settings of the analyzer. The degree of polarization (DOP) contains the critical information one needs in order to estimate coupling length h. The DOP is derived from the Stokes parameters as follows:   DOP  =                              S1          2                +                  S2          2                +                  S3          2                      S0  
The degree of polarization of the light launched by the P-OTDR source can be considered as being 100% (DOP=1) to a first approximation, since the light source is a laser. The DOP of the backscattered light from a specific position along the fiber also is equal to 1.0. However, the DOP measured by the P-OTDR will diminish if the SOP of the backscattered signal against distance varies significantly within the P-OTDR resolution, i.e., if Lb less than Lp where Lp is the P-OTDR spatial resolution and Lb is the beat length of the backscattered signal. The measured DOP against distance will therefore vary depending on the ratio between Lb and Lp. For long P-OTDR pulses (Lp greater than  greater than Lb) a strong depolarization will occur but one can still distinguish between two situations: short and long coupling length, h.
When spatial resolution is much greater than both beat length and coupling length, i.e., Lp greater than  greater than Lb and Lp greater than  greater than h, the orientations of the fast and slow axes change rapidly within the P-OTDR resolution. This makes the SOP of the backscattered signal along the pulse substantially random and the measured DOP collapses (however the average DOP does not reach zero since partial repolarization occurs on the way back; the DOP therefore tends toward ⅓).
When spatial resolution, i.e, the pulse length, is much greater than beat length, but much less than coupling length, i.e., Lp greater than  greater than Lb but Lp less than  less than h, the orientation of the birefringence axis (the fast/slow axis on the Poincare sphere) does not change within the spatial resolution and the SOP of the backscattered signal rotates rapidly around the birefringence axis. Since the P-OTDR resolution is not sufficient to follow the rapid fluctuations of the actual backscattered SOP (i.e, as would be measured using short pulses), the xe2x80x9clong pulsexe2x80x9d SOP will xe2x80x9ccollapsexe2x80x9d towards the center of the circle traced by the actual, i.e., xe2x80x9cshort pulsexe2x80x9d SOP. FIG. 1A illustrates long and short pulse measurements for the case where there is a large angle between the birefringence axis BIR and the locus of the SOP for a given distance. FIG. 1B illustrates them for the case where the angle is small.
From the xe2x80x9clong pulsexe2x80x9d or measured SOP, the xe2x80x9clong pulsexe2x80x9d or measured DOP will be measured and will tend towards the value of the cosine of the angle between the actual SOP (using short pulses) and the birefringence axis BIR. It is therefore expected that the measured DOP (using long pulses) will be anywhere between 0 and 1.0. As long as the orientation of the birefringence axis of the fiber does not change against distance (relative to an initial point), the measured DOP value will not change. If the orientations of the slow and fast axes move, the DOP value will vary. Slow fluctuations with large amplitude of the measured DOP are therefore expected on fibers with a very long coupling length h (an example of this behaviour can be seen in FIG. 2 which shows large, slow variations of DOP for the fiber section between 500 m and 1,000 m, measured by a P-OTDR using 100 ns pulses).
Huttner et al [5] proposed using these slow fluctuations of the DOP to detect the presence of long coupling length h and accordingly high-PMD fiber sections. An interesting characteristic of this technique is that xe2x80x98longxe2x80x99 P-OTDR pulses could be used. The resolution of the P-OTDR has to remain smaller than the coupling length h, but it does not have to be sufficient to discriminate the backscattering SOP fluctuation. This allows a significantly higher useful dynamic range for the P-OTDR apparatus since pulse width in the order of 100 ns to 1000 ns can be used.
Unfortunately, an important limitation of this technique arises when testing a concatenation of short fibers having very dissimilar birefringence values. If some sections have a small birefringence, such that beat length is comparable to the P-OTDR resolution, and other sections have a longer beat length, the DOP signature will exhibit slow and large fluctuations, as illustrated in FIG. 3B which shows the DOP signature of a link of 1.5 km made of 3 sections each of 500 m that exhibit dissimilar mean birefringence (0.04 ps/km, 1 ps/km, 0.04 ps/km). The signature will be very similar to the signature produced by a link made of sections that exhibit a long coupling length h, as shown in FIG. 3A, which illustrates the DOP signature of a link of 1.5 km that exhibits a large value of h (h close to 500 m). This ambiguity makes reliable interpretation of the DOP signature difficult when a link is made of a concatenation of relatively short fibers sections (for example, 2 km or less).
The present invention seeks to overcome, or at least mitigate, the limitations of the above-described prior art and/or provide an alternative.
According to one aspect of the present invention, a method of measuring characteristics of an optical path comprises the steps of:
transmitting a first series of polarized optical signal pulses into the path, each of the pulses having a length that is long compared with a beat length of the optical path and having a first state of polarization (SOP);
transmitting at least a second series of pulses into the path, the pulses of said second series each having a length comparable to that of the pulses of the first series but having a second state of polarization that differs significantly from the state of polarization of the pulses of said first series of pulses when the first and second states of polarization are represented on the Poincare sphere;
measuring, and averaging over a prescribed period of time, intensity of backscattered light corresponding to each of the first and second series of pulses;
deriving, from said intensity measurements, a DOP signature comprising a degree of polarization with respect to distance along the path for each of the first and second states of polarization; and
analyzing said DOP signatures to determine variations in polarization mode dispersion (PMD) along the length of the path.
The step of deriving the degree of polarization may comprise the steps of sampling the intensity readings for each of the states of polarization at a multiplicity of distances along the path, using the samples to compute Stokes parameters for each distance and using the Stokes parameters to compute the DOP at each of the sample distances thereby, deriving a DOP signature trace for each of the first and second states of polarization.
The step of analyzing the statistics may involve determination of differences between the DOPs, or ratios between the DOPS, or autocorrelation therebetween, frequency analysis, and so on.
Preferably the SOPs are orthogonal on the Poincarxc3xa9 sphere For example, if the first SOP is linear, the second SOP may be linear and have an orientation of 45 degrees relative to the first SOP.
Thus, the invention is predicated upon quantifying the relationship between the statistics of the backscattered DOP and the coupling length h.
One preferred approach is to use the width of the autocorrelation function of the DOP, hDOP, to quantify h since such function hDOP is directly proportional to h.
Preferably, the step of analyzing the statistics measures successive backscattered degree of polarization (DOP) traces taken for different P-OTDR source states of polarization (SOP) that are well apart on the Poincarxc3xa9 Sphere. This yields a significant improvement in the reliability for the detection of long coupling length h.
Preferably, measurements also are taken using a third input SOP, though a simplification using only two input SOPs is also possible.
Preferably, all input SOPs are mutually-orthogonal SOPs when represented on the Poincarxc3xa9 shape. For example the SOPs could be linear horizontal (0 degrees), linear diagonal (45 degrees) and circular
According to a second aspect of the invention, apparatus for measuring optical characteristics of an optical path, such as an optical fiber, comprises:
means for transmitting a first series of polarized optical signal pulses into the path, each of the pulses having a length that is long compared with a beat length of the optical path and having a first state of polarization (SOP);
means for transmitting at least a second series of pulses into the path, the pulses of said second series each having a length comparable to that of the pulses of the first series but having a second state of polarization that differs significantly from the state of polarization of the pulses of said first series of pulses when the first and second states of polarization are represented on the Poincarxc3xa9 sphere;
means for measuring, and averaging over a prescribed period of time, intensity of backscattered light corresponding to each of the first and second series of pulses;
means for deriving, from said intensity measurements, a DOP signature comprising a degree of polarization with respect to distance along the path for each of the first and second states of polarization; and
means for analyzing said DOP signatures to determine variations in polarization mode dispersion (PMD) along the length of the path.
In embodiments of either aspect of the invention, statistical analysis of the degree of polarization (DOP) of the backscattered signal advantageously may be used to detect sections of an optical transmission path for which mode-coupling behaviour (long h) leads to high PMD.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings, of a preferred embodiment of the invention, which is described by way of example only.